No, you can't, because your upstream's shortest route leads back to you and that's a loop. Any difference in route calculation between two nodes in a link-state protocol is likely to create a loop.
On 23 August 2025 17:57:10 CEST, Saku Ytti <[email protected]> wrote: >On Sat, 23 Aug 2025 at 18:54, nanog--- via NANOG <[email protected]> wrote: > >> on second thought, the real reason is that link-state protocols are >> distributed algorithms which require all nodes to execute the same algorithm >> on the same data, so there's no room to apply policy that wasn't baked into >> the design of the protocol. > >It doesn't really matter for sending direction which egress they >choose, as long as it doesn't loop. So even in this SPT future, I can >choose longer upstream over shorter by local policy, just like today. > >The big difference is, that the receiver cannot cherry pick which >prefixes to receive in which eBGP, you have to be able to receive all >prefixes on all eBGP with a given ASN. And these consistent >announcements are not today always used, and would need to be replaced >by registering multiple ASN. > > >> >> >> On 23 August 2025 16:49:27 CEST, Saku Ytti via NANOG <[email protected]> >> wrote: >> >The SPF discussion reminded me of a question I've been thinking about. >> > >> >Why do we use distance vector EGP? Why do we advertise prefixes? >> > >> >BGP made sense when we didn't have to worry about degenerates, when >> >the Internet was largely academic. Prefix is configured once to the >> >site where it exists, and no one else does anything, very optimal. >> > >> >But is that sensible today? When we have to also configure the prefix >> >out-of-band locally on every site, potentially 3 times, RPKI (RTR >> >maybe), prefix-list (for BGP) and access-list (for antispoof). So if >> >we discover ASN/Prefix association anyhow out-of-band, why do we need >> >to see +million prefixes in-band? >> > >> >What if EGP would flood link-states? What would we win? What would we lose? >> > >> >Potential wins: >> > - flooded link-states could be signed, so we could verify both >> >AS1->AS2, AS2<-AS1 link-state exists with valid signatures. You >> >couldn't hijack ASN, the entire path could be validated. >> > - initial convergence would be 50-100 times faster >> > - lot less signalling/flapping >> > - loop free alternatives for rapid convergence >> > >> >We could see some problems, for TE reasons I might advertise different >> >prefixes from different sites with the same AS. I'm not sure if that >> >is a legitimate concern, those are niche cases and for those cases we >> >could just register more ASNs and move the ASNs instead of prefixes. >> >But I'm sure there are more obvious weaknesses that don't immediately >> >spring to mind. >> > >> >-- >> > ++ytti >> >_______________________________________________ >> >NANOG mailing list >> >https://lists.nanog.org/archives/list/[email protected]/message/L2FW4MX25TFEX6IUUR5IOFQNGEVUX54T/ >> _______________________________________________ >> NANOG mailing list >> https://lists.nanog.org/archives/list/[email protected]/message/D6VUEYYE43F5NLAI2Y67QD5O5XU7NPGA/ > > > >-- > ++ytti _______________________________________________ NANOG mailing list https://lists.nanog.org/archives/list/[email protected]/message/ZDT3O7TPU3JSBMFCQIYIRHTWUDTCKOQ4/
